1. Technical Field
This invention relates to a phased array antenna arrangement with filtering to reduce grating lobes, and more particularly, to placing a filter at one of the antenna arrangment's real focal points in such a manner so as to block the grating lobes due to the array structure while allowing the central ray to pass through the filter.
2. Description of the Prior Art
Scanned reflector or lens antennas are most often proposed or used for grating lobe reduction because of their high gain, their simplicity, and their minimization of the array problem. One such type of scanned reflector antenna is disclosed in U.S. Pat. No. 3,877,031 issued to R. Mailloux et al on Apr. 8, 1975, which relates to method and apparatus for suppressing grating lobes in an electronically scanned antenna array. Grating lobe suppression is realized by adding odd mode power to the fundamental even mode power that normally drives each radiating element of the array. The odd mode power is maintained .+-.90 degrees out of phase with the even mode power at each radiating element aperture. The ratio of even mode power to odd mode power is varied as a function of main beam displacement from broadside to control the amount of grating lobe radiation. However, the scanning capability of the Mailloux et al arrangement decreases as the main reflector gain is increased. Moreover, the Mailloux et al arrangement has a low aperture efficiency and so must be large as compared with an efficiently illuminated aperture.
Another method of grating lobe reduction is disclosed in U.S. Pat. No. 4,021,812 issued to A. Schell et al on May 3, 1977, which relates to suppression of side lobes and grating lobes in directional beam forming antennas by the use of a spatial filter. The filter consists of flat layers of high dielectric-constant material separated by air or other low dielectric-constant materials. The filter is placed directly over the feed array, the dielectric-constant and thickness values thereby effecting full transmission of beam power in a selected beam direction so as to suppress side and grating lobes.
Grating lobe reduction may also be obtained by strategically arranging the array elements. An example of this is contained in the article entitled "Grating-Lobe Suppression in Phased Arrays by Subarray Rotation" by V. Agrawal in Proceedings of the IEEE, Vol. 66, No. 3, March 1978 at pp. 347-349. In this method, the array is divided into equal subarrays which are physically rotated with respect to each other by specified angles. As a result, the grating lobes, which remain at the same angular distance from the main beam, are multiplied in number by the number of subarrays while their amplitude is divided by the same number. Therefore, in a combined pattern, the main beams of the subarrays will add, while the grating lobes of each subarray will be positioned over a null of another of the remaining subarrays.
The problem remaining in the prior art is to achieve grating lobe suppression in phased array systems by utilizing a simplified array arrangement without excessive degradation in performance of the system.